Pressure sensitive adhesive (PSA) formulations based on e.g. aqueous acrylic and carboxylated styrene/butadiene co-polymers usually contain a tackifying resin to impart to the final adhesive formulation, specific properties such as tackiness, peel-resistance and ease of surface wetting.
As is well known in the art, there are two major classes of tackifying resins, namely rosin and derivatives thereof, and hydrocarbon resins. The rosin and derivatives class are obtained from pine trees, and include rosins, modified rosins, and various rosin derivatives, for example rosin esters. The hydrocarbon resin class include low molecular weight polymers produced from a wide variety of monomers mainly based on C5 to C9 hydrocarbons derived primarily from petrochemical sources.
In aqueous PSA formulations, tackifying resins are commonly added in the form of an aqueous dispersion or emulsion, firstly made by melting the resin and then adding a surfactant and water to form a so-called oil-in water emulsion/dispersion where the resin is the dispersed phase and water is the continuous phase. The purpose of surfactants is twofold, namely; a) to decrease the interfacial tension between the water and the resin, hence to aid the dispersion process by creating very small resin particles, and b) to provide long-term stability to the so-obtained resin dispersion. There are a large variety of known surfactants, but they are not all suitable for use in tackifying resins. In particular, the use of many surfactants can result in the tackifying resin dramatically reducing the cohesive strength of the adhesive.